The problems associated with the lubrication of automatic and manual transmissions and the operation of hydrualic fluid systems are well known to those skilled in the art. For example, in the lubrication of transmissions, proper fluid viscosity at both low and high temperatures is essential to successful operation. Good low temperature fluidity eases cold weather starting and insures that the hydraulic control system will properly "shift gears". High viscosity at elevated temperatures insures pumpability and the satisfactory functioning of converters, valves, clutches, gears and bearings.
In the operation of hydraulic fluid systems, proper fluid viscosity at both low and high temperatures is essential to successful operation. High temperature viscosity retention is beneficial in lubrication, contributes to streamline flow and reduces leakage. Good low temperature fluidity provides rapid control action, less heating loss and lower pressure drop.
These conflicting fluidity requirements call for a product that exhibits the following characteristics:
(A) high temperature viscosity retention,
(B) low temperature fluidity, and
(C) shear stability.
In order to prepare lubricants having these characteristics, it has become common practice to add a variety of chemicals to the oil. For example, in order to meet the viscosity requirements, compositions have been added to the oils which are characterized by relatively small change in their viscosity with changing temperature, and they are commonly graded according to SAE standards according to the viscosities at low (e.g., 0.degree. F.) and at high temperatures (e.g., 210.degree. F.). As a result of the incorporation of such additives, the lubricating oils are often referred to as being "multi-graded". In terms of widely aocepted concepts, such multi-graded lubricants have the desirable properties are being able to function immediately, through cold, upon being put into service, and to continue to function satisfactorily as they become heated during operation.
Although chemical compositions have been developed which improve the viscosity characteristics of lubricating oil, it is often desirable to further improve the low temperature characteristics by including compositions which function as fluidity modifiers at low temperatures. Fluidity modifiers are capable of lowering the viscosity of a lubricating oil at low temperatures generally by retarding the formation of undesirable microcrystalline wax substances.
In addition to the above improvements, it is desirable, if not necessary, that lubricating compositions especially designed for use as transmission fluids and hydraulic fluids exhibit shear stability. Shear stability means that the lubricating oils will not degrade or lose their desirable viscosity characteristics as a result of the shearing forces encountered during their use. Lubricating oil compositions exhibiting desirable shear stability will be found to generally have the viscosity within 85-95% of their original viscosity after a number of hours, (e.g., 100 hours) of service. It has been recognized that many ordinary viscosity index improvers commonly added to crankcase lubricating oils, such as high molecular weight polyisobutene and polyacrylates, do not possess the desired shear stability for use in improving the viscosity characteristics of transmission fluids and hydraulic fluids.
It now has been found that multi-grade lubricants exhibiting improved shear stability can be made by utilizing the compositions of the present invention. These lubricants are particularly useful in providing transmission fluids and hydraulic fluids.